Liquid crystal composition

ABSTRACT

Liquid crystal compositions containing at least one compound of the formula I 
     
         R-Dio-Ph-CN                                                I 
    
     wherein R is alkyl of 2 to 5 carbon atoms, Dio is trans-1,3-dioxane-2,5-diyl and Ph is 1,4-phenylene, and at least one compound of each of the following groups A to D: 
     
         R.sup.1 -Cy-Ph-R.sup.2                                     A 
    
     
         R.sup.3 -Cy-Ph-Ph-R.sup.4                                  B 
    
     
         R.sup.5 -Cy-Ph-CN                                          C 
    
     
         R.sup.6 -Cy-COO-Ph-O-R.sup.7                               D 
    
     and, in addition in a total amount of 10 to 26% by weight at least five compounds of the formula E 
     
         R.sup.8 -Pyr-Ph-O-R.sup.9                                  E 
    
     or at least one compound of each of the following groups F to K: 
     
         R.sup.10 -Ph-COO-Ph-R.sup.11                               F 
    
     
         R.sup.12 -Cy-COO-Ph-Ph-CN                                  G 
    
     
         R.sup.13 -Ph-Ph-COO-Ph-Ph-CN                               H 
    
     
         R.sup.14 -Ph-COO-Ph-COO-Ph-R.sup.15                        J 
    
     
         R.sup.16 -Ph-Ph-COO-PhF-R.sup.17                           K 
    
     result in surprisingly steep electrooptical characteristics and surprisingly low viewing-angle dependence of the contrast at simultaneously acceptable threshold voltages for commercially available drivers and multiplex ratios up to 1:100 when used in matrix displays.

BACKGROUND OF THE INVENTION

The invention relates to liquid crystal compositions suitable for use inelectro-optical matrix displays of the twisted nematic cell type, and toan electro-optical matrix display using the liquid crystal compositionmentioned above.

There is still a great demand for liquid crystal compositions (LCcompositions) suitable for matrix displays having an electroopticalcharacteristic (contrast/voltage curve) as steep as possible andsimultaneously a low threshold voltage. In such matrix displays, each"display point" of an electrode grid can be selectively triggered byapplying a voltage to a first electrode layer, consisting of amultiplicity of horizontral rows of conductors, and to another electrodelayer, consisting of a multiplicity of vertical rows of conductors(multiplex drive). The highr the multiplex ratio and the lower themaximum voltage of the available drivers, the lower the thresholdvoltage of the LC composition has to be. A disadvantage of these matrixdisplay elements is the partial activation of display points in theimmediate vicinity of a triggered display point, which partialactivation is called "crosstalk" and reduces the contrast of the displayin an undesirable manner. The steeper the electro-optical characteristicof the LC composition used in a display element of this type, the lesssuch crosstalk is then observed. As a rule, the steepness of theelectro-optical characteristic of a LC composition is given as the ratioof the control voltages V which must be applied to a given displayelement in order to obtain 10% of the maximum contrast (V₁₀) and 50% ofthe maximum contrast (V₅₀).

The steepness of the electro-optical characteristic is the greater, thesmaller the ratio ##EQU1## Till today the steepness of theelectro-optical characteristic can only be determined by experiment,because all theoretical predictions ended up to now in rather wrongresults.

Besides of a decrease of the contrast with increasing multiplex ratio,there is also a strongly increasing viewing-angle dependence of thecontrast. This dependence is defined as ##EQU2## wherein V_(x),y,z isthe applied control voltage at x % contrast, viewing angle y andtemperature z° C.

The smaller β, the better is the optical performance of the matrixdisplay.

SUMMARY OF THE INVENTION

It has now been found that LC compositions according to the inventionresult in surprisingly steep electro-optical characteristics andsurprisingly low viewing-angle dependence of the contrast atsimultaneously acceptable threshold voltages for commercially availabledrivers and multiplex ratios up to 1:100 when used in matrix displays.

The invention thus relates to a liquid crystal composition,characterized in that it contains at least one compound of the formula I

    R-Dio-Ph-CN                                                I

wherein R is alkyl of 2 to 5 carbon atoms, Dio istrans-1,3-dioxane-2,5-diyl and Ph is 1,4-phenylene, and at least onecompound of each of the following groups A to D:

    R.sup.1 -Cy-Ph-R.sup.2                                     A

wherein R¹ and R² are each alkyl of 3 to 5 carbon atoms, Cy istrans-1,4-cyclohexylene and Ph is 1,4-phenylene,

    R.sup.3 -Cy-Ph-Ph-R.sup.4                                  B

wherein R³ and R⁴ are each alkyl of 2 to 5 carbon atoms and Cy and Phhave the meanings indicated above,

    R.sup.5 -Cy-Ph-CN                                          C

wherein R⁵ is alkyl of 2 to 5 carbon atoms and Cy and Ph have themeanings indicated above,

    R.sup.6 -Cy-COO-Ph-O-R.sup.7                               D

wherein R⁶ and R⁷ are each alkyl of up to 5 carbon atoms and Cy and Phhave the meanings indicated above, and, in addition in a total amount of10 to 26% by weight at least five compounds of the formula E

    R.sup.8 -Pyr-Ph-O-R.sup.9                                  E

wherein R⁸ and R⁹ are each alkyl of 5 to 11 carbon atoms, Pyr ispyrimidine-2,5-diyl and Ph has the meaning indicated above,

or at least one compound of each of the following groups F to K:

    R.sup.10 -Ph-COO-Ph-R.sup.11                               F

wherein R¹⁰ and R¹¹ are each alkyl of 1 to 7 carbon atoms, Ph has themeaning indicated above,

    R.sup.12 -Cy-COO-Ph-Ph-CN                                  G

wherein R¹² is alkyl of 1 to 3 carbon atoms, and Cy and Ph have themeaning indicated above.

    R.sup.13 -Ph-Ph-COO-Ph-Ph-CN                               H

wherein R¹³ is alkyl of 6 to 8 carbon atoms and Ph has the meaningindicated above.

    R.sup.14 -Ph-COO-Ph-COO-Ph-R.sup.15                        J

wherein R¹⁴ and R¹⁵ are each alkyl of 1 to 5 carbon atoms and Ph has themeaning indicated above,

    R.sup.16 -Ph-Ph-COO-PhF-R.sup.17                           K

wherein R¹⁶ and R¹⁷ are each alkyl of 5 to 7 carbon atoms, PhF isfluorinated 1,4-phenylene and Ph has the meaning indicated above,

and to an electro-optical display device, characterized that it containsa liquid crystal composition according to the above.

The LC compositions according to the invention preferably contain atleast two compounds of the formula I preferably in a total amount of 10to 25, preferably 11,5 to 20% by weight.

The LC compositions according to the first aspect of the inventionpreferably contain only one compound of the formula A preferably in anamount of 5 to 15% by weight, preferably at least two compounds of theformula B preferably in an total amount of 15 to 25% by weight andpreferably only one compound of the formula C preferably in an amount of5 to 10% by weight. These LC compositions according to the inventionpreferably contain at least three compounds of the formula D preferablyin a total amount of 10 to 26% by weight.

R⁶ preferably is straight chain alkyl of 3 to 5 carbon atoms. R⁷preferably is methyl.

These LC compositions according to the invention preferably contain fivecompounds of the formula E preferably in a total amount of 20 to 25% byweight.

R⁸ preferably is n-hexyl.

The LC compositions according to the second aspect of the inventionpreferably contain at least six compounds of the formula F preferably ina total amount of 50 to 65% by weight and preferably only one compoundof the formula G preferably in an amount of 5 to 15% by weight, R¹² ispreferably ethyl.

These LC compositions according to the invention preferably contain onlyone compound of the formula H preferably in an amount of 1 to 5% byweight. R¹³ is preferably n-heptyl.

These LC compositions according to the invention preferably contain onlyone compound of the formula J preferably in an amount of 3 to 7% byweight.

R¹⁴ and R¹⁵ are preferably n-propyl.

These LC compositions according to the invention furthermore preferablycontain only one compound of the formula K preferably in an amoount of 7to 11% by weight.

R¹⁶ is preferably n-heptyl and R¹⁷ is preferably n-pentyl and PhF ispreferably 2-fluoro-1,4-phenylene.

The preparation of the LC compositions according to the invention iscarried out in a manner which is conventional per se. The singlecomponents are known from the prior art. As a rule, the desired quantityof the components used in a smaller quantity is dissolved in thecomponent(s) representing the main constituent, advantageously at anelevated temperature. If the temperature selected here is above theclearing point of the main constituent, the completeness of the solutionprocess can be observed with particular ease.

It is also possible, however, to mix solutions of the components in asuitable organic solvent, for example acetone, chloroform or methanol,and, after thorough mixing, to remove the solvent again, for example bydistillation under reduced pressure. Of course, it is necessary in thisprocedure to take care that no impurities or undesired doping substancesare introduced by the solvent.

The examples which follow are intended to explain the invention withoutrestricting it. Parts or percentage figures denote parts by weight orpercent by weight respectively.

EXAMPLE A

A LC composition consisting of

7% 2-p-cyanophenyl-5-propyl-1,3-dioxane,

7% 2-p-cyanophenyl-5-butyl-1,3-dioxane,

6% 2-p-cyanophenyl-5-pentyl-1,3-dioxane,

8% p-trans-4-propylcyclohexyl-benzonitrile,

12% 4-ethyl-4'-(trans-4-propylcyclohexyl)-biphenyl,

11% 4-ethyl-4'-(trans-4-pentylcyclohexyl)-biphenyl,

3% 2-p-pentoxyphenyl-5-hexylpyrimidine,

3% 2-p-hexoxyphenyl-5-hexylpyrimidine,

4% 2-p-heptoxyphenyl-5-hexylpyrimidine,

6% 2-nonoxyphenyl-5-hexylpyrimidine,

6% 2-p-undecoxyphenyl-5-hexylpyrimidine,

7% trans-4-propylcyclohexancarboxylic acid-(p-methoxyphenylester),

5% trans-4-butylcyclohexancarboxylic acid-(p-methoxyphenylester),

4% trans-4-pentylcyclohexancarboxylic acid-(p-methoxyphenylester) and

11% trans-1-p-propylphenyl-4-pentylcyclohexane

has a clear point of 61° C. and a viscosity of 31 mPa.s at 20° C.

EXAMPLE B

A LC composition consisting of

8.2% 4-(5-ethyl-1,3-dioxane-2-yl)benzonitril

3.3% 4-(5-butyl-1,3-dioxane-2-yl)benzonitril

13.7% 4-pentylphenyl 4-methylbenzoate

13.8% 4-pentylphenyl 4-propylbenzoate

7.0% 4-pentylphenyl 4-pentylbenzoate

4.0% 4-propylphenyl 4-heptylbenzoate

10.0% 4-pentylphenyl 4-heptylbenzoate

12.5% 4-heptylphenyl 4-heptylbenzoate

11.0% 4'-cyano-4-biphenylyl 4-ethyl-1-cyclohexylcarboxylate

2.5% 4'-cyano-4-biphenylyl 4'-heptyl-4-biphenylcarboxylate

5.0% 4-propylphenyl 4-(4-propylbenzoyloxy)benzoate

9.0% 2-fluoro-4-pentylphenyl 440 -heptyl-4-biphenylcarboxylate

has a clear point of 62° C. and a viscosity of 53m PaS at 20° C.

The properties of the LC compositions described above (Examples A and B)in a twisted nematic cell are compared in Table 1 with those of acommercially available LC composition ZLI-2448-000 (E. Merck, Darmstadt,Germany).

From the results given in Table I it is clearly noted that the LCcompositions according to the invention (Examples A and B) have muchsteeper electro-optical characteristics (γ).

In addition there is a clear improvement of the viewing-angle dependenceof the contrast (β). The achieved values of γ and β combined with thelow threshold voltage V₉₀ allow to realize a dot-matrix twisted nematicliquid crystal display device addressed by multiplex ratios of 1:100,using commercially available driver circuits.

These devices show excellent display performances and practicallyacceptable operating life-time.

                  TABLE I                                                         ______________________________________                                                        Conven-  Example A Example B                                                  tional   of this   of this                                    Item            example  invention invention                                  ______________________________________                                        V.sub.10,0,25 [Volt]                                                                          2,19     1,83      1,90                                       (threshold voltage)                                                            ##STR1##       15,9     12,8      13,0                                       100 [%]                                                                       (steepness of the electro-                                                    optical characteristic)                                                        ##STR2##       23,1     18,5      18,0                                       100 [%]                                                                       (viewing-angle dependence)                                                    k.sub.33 /k.sub.11 (25° C.)                                                            1,12     0,82      0.92                                       Ratio of bend/splay elastic                                                   constants, which mainly                                                       determines the steepness of                                                   the electro-optical character-                                                istic. The smaller the ratio                                                  k.sub.33 /k.sub.11, the steeper the                                           electro-optical characteristic.                                               ______________________________________                                    

We claim:
 1. A liquid crystal composition containing about 10-25 wt% ofat least one compound of the formula I

    5-R-Dio-Ph-CN                                              I

wherein R is alkyl of 2 to 5 carbon atoms, Dio istrans-1,3-dioxane-2,5-diyl and Ph is 1,4-phenylene, and (a) at least onecompound of each of the following groups A to D:

    R.sup.1 -Cy-Ph-R.sup.2                                     A

wherein R¹ and R² are each alkyl of 3 to 5 carbon atoms, Cy istrans-1,4-cyclohexylene and Ph is 1,4-phenylene,

    R.sup.3 -Cy-Ph-Ph-R.sup.4                                  B

wherein R³ and R⁴ are each alkyl of 2 to 5 carbon atoms and Cy and Phhave the meanings indicated above,

    R.sup.5 -Cy-Ph-CN                                          C

wherein R⁵ is alkyl of 2 to 5 carbon atoms and Cy and Ph have themeanings indicated above,

    R.sup.6 -Cy-COO-Ph-O-R.sup.7                               D

wherein R⁶ and R⁷ are each alkyl of up to 5 carbon atoms and Cy and Phhave the meanings indicated above, and, in addition, in a total amountof 10 to 26% by weight, at least five compounds of the formula E

    5-R.sup.8 -Pyr-Ph-O-R.sup.9                                E

wherein R⁸ and R⁹ are each alkyl of 5 to 11 carbon atoms, Pyr ispyrimidine-2,5-diyl and Ph has the meaning indicated above, theremainder being the amount of compounds A to D; or (b) 25-90% by weightin total of at least one compound of each of the following groups F toK;

    R.sup.10 -Ph-COO-Ph-R.sup.11                               F

wherein R¹⁰ and R¹¹ are each alkyl of 1 to 7 carbon atoms, Ph has themeaning indicated above,

    R.sup.12 -Cy-COO-Ph-Ph-CN                                  G

wherein R¹² is alkyl of 1 to 3 carbon atoms, and Cy and Ph have themeaning indicated above,

    R.sup.13 -Ph-Ph-COO-Ph-Ph-CN                               H

wherein R¹³ is alkyl of 6 to 8 carbon atoms and Ph has the meaningindicated above,

    R.sup.14 -Ph-COO-Ph-COO-Ph-R.sup.15                        J

wherein R¹⁴ and R¹⁵ are each alkyl of 1 to 5 carbon atoms and Ph has themeaning indicated above,

    R.sup.16 -Ph-Ph-COO-PhF-R.sup.17                           K

wherein R¹⁶ and R¹⁷ each alkyl of 5 to 7 carbon atoms, PhF isfluorinated, 1,4-phenylene ##STR3## and Ph has the meaning indicatedabove.
 2. In an electro-optical display device based on a liquid crystalcomposition, the improvement wherein the composition is that of claim 1whereby the device has a steep electro-optical characteristic (γ) and awide viewing angle dependence of the contrast (β).
 3. A composition ofclaim 1, comprising I and (a).
 4. A composition of claim 1, comprising Iand (b).